Since separation/concentration of liquid mixtures by a reverse osmosis membrane method is an energy-saving method and does not cause the phase changes in the state of substances as compared with a separation technique such as distillation, it has been widely utilized in many fields including a food field such as concentration of fruit juice and separation of beer enzyme; a water purification field such as production of drinking water, industrial water, etc. by desalination of seawater and saline water, production of ultrapure water in electronic industry and production of sterile water in pharmaceutical industry and medical field; and recovery of organic substances from industrial wastewater. Treatment of water by a reverse osmosis membrane has become firmly established as an indispensable process supporting the most leading-edge technology.
For example, desalination of seawater and saline water using a reverse osmosis membrane is a clean process and, as compared with thermal desalination method and electrodialysis method, it is advantageous in view of conservation of energy, low cost and simplicity in operation and has produced big results up to now. Particularly due to the fact that a hollow fiber type reverse osmosis membrane can make the membrane area per module large in spite of its small volume of permeable water per unit membrane area as compared with a spiral type reverse osmosis membrane, it has advantages that the permeable water volume can be made large as a whole and that volume efficiency is very high whereby it has been abundantly adopted.
Generally, the hollow fiber type reverse osmosis membrane as mentioned above is manufactured in such a method that a dope containing cellulose acetate is prepared as a polymer material, extruded from a spinneret into air, coagulated in aqueous solution, washed with water and shrunk by annealing by hot water. For example, Examples in the Patent Document 1 disclose a reverse osmosis membrane prepared in such a manner that a dope containing cellulose triacetate as a polymer material is extruded, coagulated, washed with water and annealed by hot water of 85° C. for 20 minutes under no tension. According to the data of the Examples, it is shown that, when 0.2% aqueous solution of sodium chloride was used as a supplying water and measurement was conducted under the pressure of 30 kg/cm2, the filtration rate and NaCl rejection rate were 230 L/m2/day and 99.85% (Example 1); 245 L/m2/day and 99.87% (Example 3); or 250 L/m2/day and 99.84% (Example 4), respectively. Since the filtration rate depends upon the pressure, although the NaCl rejection rate is not so much affected, the filtration rate lowers to an extent of 120 L/m2/day being about one half if the reverse osmosis membrane of the above Examples is measured at 15 kg/cm2 which is one half of the above pressure. Thus, although the conventional reverse osmosis membrane as in the Patent Document 1 can achieve high desalination efficiency because the membrane shrinkage was made large by the annealing treatment by hot water of high temperature, it has a problem that the water permeability lowers when used under low pressure.
Examples of an art where both water permeability and rejection rate are intended to be kept in high level in a reverse osmosis membrane include Patent Documents 2 and 3. The Patent Document 2 discloses an art concerning a hollow fiber type reverse osmosis membrane module which is utilized for the separation of solid or solute from a liquid mixture. However, according to the properties of the hollow fiber membrane using cellulose triacetate shown in Table 1 of the Patent Document 2, the filtration rate (FR1) measured under operation pressure of 55 kg/cm2 is 22.6 to 91.5 L/m2·day and no high water permeability can be achieved.
Patent Document 3 discloses an art concerning a flat sheet type composite reverse osmosis membrane which is equipped, on a microporous support, with an active layer (thin membrane, skin layer) containing interfacial polymerized polyamide as a main component and which has both high salt rejection rate and high permeability. According to the description of Example 1, the reverse osmosis membrane disclosed in the Patent Document 3 has permeation flow rate of 1.0 m3/m2·day (1000 L/m2·day) when measured under an operation pressure of 7.5 kg/m2. However, in this reverse osmosis membrane, aqueous amine solution is made to contact to a polysulfone supporting membrane and then aqueous acid chloride solution is made to contact thereto whereby an interfacial polymerization polyamide layer is formed. Thus, complicated manufacturing steps are needed. Further, a membrane comprising such materials has a problem in terms of resistance to chlorine and has a disadvantage that the cleaning chemicals which can be used therefor are limited.
On the other hand, from recent users attaching great importance to economy, there has been a strong demand for cost reduction in the production of water by a hollow fiber type reverse osmosis membrane. For example, in reverse osmosis methods, cost for power (electric power cost for high-pressure pump) occupies almost one half of the cost for production of water and, when a conventional high pressure reverse osmosis membrane for seawater desalination is used for the treatment of wastewater, it is not possible to suppress the water production cost since the electric power cost is high. Even when a low pressure pump is used and, further, the conventional low pressure reverse osmosis membrane is used for reducing the power cost, no high water permeability is achieved under the current state and, in conclusion, it is not possible to suppress the cost for water production.
As such, it is the current status that there is no hollow fiber type reverse osmosis membrane based on cellulose acetate which can achieve both water permeability and rejection rate in high level at low running cost.